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Old 02-18-2021, 10:33 PM   #1
alexiskai
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Default Stud boss height for stock heads

I'm doing some research for my cylinder head install video and I'm interested in what people say about installing the studs. Notably, some folks say you should hand tighten the studs in the block and then back them out one full turn (Rich Fallucca of AER says this, you can watch him say it right here).

Fallucca goes on to say, "this gives you about one and a half threads above the top of the nut. If the studs today are screwed all the way down into the engine block, and you put the nuts on there, the top of this stud will be about a thread and a half below the top of the nut. We're looking to have it about a thread and a half above the top of the nut, so we unscrew the studs. It won't hurt anything."

This confused me. Are "the studs today" shorter than the OEM studs? This seems relatively easy to fix. But then someone suggested to me that the actual problem is that many HC heads are taller than the stock head, measured at the height of the stud boss above the deck. I don't have a collection of HC heads, so I can't check this.

Bottom line, I'm trying to give good advice to the folks watching the video, some of whom may have out-of-spec heads. What I'd like to say is something along the lines of "If your stud boss height is greater than X, you'll need to unscrew your stud one turn to ensure you have adequate thread engagement on top."

Does anyone know the spec for the height of the original head as measured at the stud boss? Anyone know whether the popular HC heads like Snyder's are taller than the spec? I know they're under-size in terms of overall length.
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Old 02-19-2021, 09:23 AM   #2
Jack Shaft
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Default Re: Stud boss height for stock heads

A stud or bolt is only as strong as its width, burying threads beyond its width does not add to its grip strength.

The same reason you don't use cap screws for head bolts is used to explain installing studs finger tight, Ford grey iron casting does not support undue thread stress, so no undue load on the block threads prior to torqueing assembly.

Ford manufacturing precision and interchangeability of parts is often taken for granted, it truly is a marvel when you consider it..5 million units produced almost complete interchangeability,, thank CE Johannson for this. High compression heads are not held to Ford's rigid standards, and some like the Brierley head introduce a taller design to improve cooling and performance, requiring longer studs. Performance modifying requires flexibility..
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Old 02-19-2021, 10:32 AM   #3
alexiskai
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Default Re: Stud boss height for stock heads

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Originally Posted by Jack Shaft View Post
A stud or bolt is only as strong as its width, burying threads beyond its width does not add to its grip strength.
The technical way of saying this is that the strength of a steel-to-steel joint increases as the length of thread engagement increases, up to one nominal diameter (e.g., for a 7/16"-14 stud, 7/16" depth of tapping or about six full turns). Beyond that there is no appreciable increase in strength.

Although this is correct, there are reasons to favor inserting the stud to the maximum practical depth. First, the beginning and end of the threaded engagement is chamfered the top of the internal threads are chamfered in the block and the bottom of the external threads are chamfered on the stud so there's minimal thread engagement in that section of the joint. Second, it's safe to assume the block threads are degraded to some degree, so only a proportion of the threaded engagement is taking the load.

If the effective loss of engaged length from these two factors, plus the amount you're backing the stud out of the block, were to exceed 0.31", you'd be below the one-nominal-diameter threshold.

So I agree there's a margin for error in backing out the stud, but because we don't know how much the internal threads are degraded, I don't think we should be backing out the stud unless it's required to properly clamp a head that's taller than stock. My question is about how an individual should go about calculating whether this is required on their engine.
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Old 02-19-2021, 11:18 AM   #4
Jack Shaft
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Default Re: Stud boss height for stock heads

In the matter of degraded threads a thread compound can mitigate the factor to a degree.RTV compounds can do double duty,seal coolant migration as well as provide nominal thread support.In my opinion I want the stud in block at no load,so I can control the load on the block threads through the smooth application of torque.Bear in mind,the torque value applied does not put the stud in fully in tension,it is more of a means to set the cylinder head assembly at a consistent compression when cold.
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Old 02-19-2021, 01:33 PM   #5
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Default Re: Stud boss height for stock heads

The original stud boss height is 2-7/32". That does not include the head gasket thickness.
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Old 02-19-2021, 03:22 PM   #6
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Default Re: Stud boss height for stock heads

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Quote:
Originally Posted by nomadpsd View Post
The original stud boss height is 2-7/32". That does not include the head gasket thickness.
Thanks nomad, that's very close to what I was getting with digital calipers on my old stock head. Good to know.
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Old 02-19-2021, 06:23 PM   #7
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Default Re: Stud boss height for stock heads

Grip length of a bolt or stud thread engagement for maximum strength is 1.5 times it's diameter.
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